The kinetics of grape aromatic precursors hydrolysis at three different temperatures

Abstract

In neutral grapes, it is known that most aroma compounds are present as non-volatile precursors. There is strong evidence that supports the existence of a connection between the content of aroma precursors in grapes and the aromatic quality of wine1. Harsh acid hydrolysis is considered the better way to reveal the aroma potential of winemaking grapes, because transformations taking place during fermentation include relevant chemical rearrangements in acid media that are better predicted by acid hydrolysis2. The aim of the present work is to establish a methodology to evaluate the aromatic potential of the grape from the acid hydrolysis in anoxic conditions of its aromatic and phenolic fraction.
In this work, firstly two different samples of Grenache grapes aromatic and phenolic fraction (PAF) were extracted, followed by acid hydrolysis under strict anoxic conditions based on a previously developed methodology3. These PAFs were reconstituted in model wine and aged in duplicate under anoxic conditions at three temperatures: 75, 50 and 35 ºC. The aged model wines were collected at different sampling times 75 ºC (1h, 2h, 6h, 12h, 24h, 48h, 96h), 50 ºC (0.5, 1, 2, 5, 7, 10 and 14 weeks) and at 35ºC (0.5, 1, 5, 3.5, 6, 9 and 12 months).
Hydrolysates were extracted and analyzed by two different analytical methods: esters, free norisoprenoids, terpenoids, phenols, lactones, vanillins and cinnamates were analyzed by SPE-GC-MS4, while varietal thiols were analyzed by LC-QqQ-MS.
The hydrolysates obtained at 50 and 75ºC present sensory profiles congruent with olfactory nuances of unoxidized wine. In fact, the absolute concentration values ​​found for terpenes, lactones and norisoprenoids are within the normal values ​​expected in a wine, except for TDN, which appears in large quantities. However, phenols, derivatives of vanilla and varietal thiols especially 3-mercaptohexanol appear in much higher amounts than would be expected in a Grenache wine, possibly because this type of hydrolysis is capable of release a major part of aromatic potential of grape. Very few differences are observed in the hydrolysis profiles between the two samples. The hydrolysis profile at the same temperature is similar between the samples in most cases even though different amounts of volatiles are obtained. All compounds seem to hydrolyze following two types of behavior that can be explained by the combination of two phenomena: the generation of volatiles (hydrolysis and rearrangements) and the subsequent degradation at wine pH. For those compounds with congruent evolutions at the three different temperatures, a model able to predict the evolution of varietal volatiles at room temperature will be presented.